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I think these papers may be of help to you:

Rahman, Ashfaqur, et al. "An image based approach to compute object distance." International Journal of Computational Intelligence Systems 1.4 (2008): 304-312.

and

section 11 of

Sharma, Sachin Umesh, and Dharmesh J. Shah. "A Practical Animal Detection and Collision Avoidance System Using Computer Vision Technique." IEEE Access 5 (2017): 347-358

The latter paper has done this:

• Image resolution is 640 × 480

• X range is 0 to 640

• Y range is 0 to 480

Let the right bottom coordinate of the detected cow be (x, y). Then the distance of cow from the lower edge (car/camera) is 480 – y.

They have then used a formula derived from the first paper to convert distance in pixels to meters